OFFSET
0,4
LINKS
Wikipedia, Permutation
EXAMPLE
In the n=3 case:
| sigma(1),sigma(2),sigma(3)
----+---------------------------
1 | [1, 2, 3]
2 | [2, 3, 1]
3 | [3, 1, 2]
MATHEMATICA
Table[ Count[ Length@ Union[# Range@ n] & /@ Permutations@ Range@ n, n], {n, 0, 9}] (* Giovanni Resta, Mar 09 2020 *)
PROG
(Ruby)
def A(n)
(1..n).to_a.permutation.select{|i| (1..n).map{|j| i[j - 1] * j}.uniq.size == n}.size
end
def A333083(n)
(0..n).map{|i| A(i)}
end
p A333083(9)
(PARI) a(n) = {my(nb=0); forperm([1..n], p, if (#Set(vector(n, k, k*p[k])) == n, nb++); ); nb; } \\ Michel Marcus, Mar 09 2020
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Seiichi Manyama, Mar 07 2020
EXTENSIONS
a(13)-a(15) from Giovanni Resta, Mar 09 2020
STATUS
approved